Modulation



G, L. UssELMAN MODULAT-I ON Filed Oct. l, 1955 3 Sheets-Sheet l ATTORNEY G.V L. USSELMAN .Ime 27, l1939.

MODULATIOE Filed Oct. l, 1955 3 Sheets-Sheet 2 BY l ATTORNEY G. L. UssELMAN -2,l63,7l9

MODULATION I Filed Oct. l, 1955 3 Sheets-Sheetv 3 June 27, 1939.

" modulation present in the signal.

Patented June 27, 1939 PATENT OFFICE MODULATION George L. Usselman, Port Jeerson, N. Y., as-

signor to Radio Corporation of America, a corporation of Delaware Original application July 23,

Patent No. 2,075,071, dated March 624,258, now

1932, Serial No.

30, 1937. Divided and this application October 1, 1935, Serial No. 43,013 v 19 Claims.

'I'he present invention relates to an improved method of and means for producing phase or amplitude modulated signals, or a combination of both, means for varying the phase or ampli tude or both of radio waves or oscillations in accordance with the signals to be transmitted, and means for regulating during operation of the transmitter the amount of phase amplitude modulation or both accomplished. The present application is a division of my U. S. Appln. Ser. No. 624,258, filed July 23, 1932, Patent #2,075,071 dated March 30, 1937.

It has been discovered recently that most amplitude modulated transmitters have some phase This in bad cases causes very objectionable distortion in the signal.

The present invention involves means heretofore unknown in the art for balancing out any phase modulation inherently produced in amplitude modulating a carrier wave at signal frequency.

It is also known that in accomplishing phase or frequencymodulation of a carrier some amplitude modulation thereof is accomplished.

- 'I'his amplitude modulation results in distortion of the signal in the receiver. f

The present invention involves means unknown heretofore in the art for balancing out any amplitude modulation or extraneous hum .inherently produced in accomplishing phase modulation of a carrier.

In phase modulated transmitters, as disclosed in United States Appln. Ser. No. 602,487, iiled April 1, 1932, Patent #2,049,143 dated July 28, 1936, and United States Appln. Ser. No. 607,932, filed April 28, 1932, Patent #2,036,164 dated March 31, 1936-, phase modulation of a high frequency carrier in accordance withV the signal to be transmitted has been accomplished.

In the first described application phase rotation of the carrier wave relative to the normal wave is accomplished by the use of a phase retarding inductance and phase advancing capacity in the grid 'circuits of the symmetrical modulator tubes.

In the second referred to application, phase modulation is accomplished by the use of lines of a different electricallength connecting the oscillation source substantially cophasally to the grids of a pair of tubes. One of the lines includes variable means, as, for example, a trombone tuning means.

In the above referred to applications in general the signal frequencies are applied to the impedances of the tubes so that a phase shift, the amount of which is determined by the amplitude of the signal, appears in the energy repeated in the tubes and consequently appears in the output of the tubes.

In practice, it is often desirable to have a transmitter by means of which the degree of phase modulation effected in accordance with the signal may be varied during the operation of the transmitter so that signalling may continue during the adjustment of the degree of phase shift.

One of the main objects of the present invention is to provide a new and novel arrangement by means of which phase modulation may be accomplished and the degree of modulation may be varied during operation of the modulating means.

This is accomplished in accordance with the present invention by feeding unmodulated carrier n circuit into which the phase modulated energy is fed and regulating the amount of unmodulated energy delivered to the tank circuit thereby regulating the degree to which the combined energy in the tank circuit is modulated in phase.

- Under other conditions transmit by means of the modulated waves and waves simultaneously so be sent out at the same either wave separately.

In its broadest aspect, this is accomplished by applying phase modulation to a carrier at signal frequency and amplitude modulations to the carrier at signal frequency simultaneously or separately, and feeding the combined energy or the energy from one of the modulators to a tank circuit, from which it may be utilized.

Another object of the present invention is to provide means whereby the hum and extraneous amplitude modulation, which may be inherently present in a phase modulator, may be corrected, compensated, or otherwise removed from the energy resulting from the phase modulation which is to be transmitted. 'Ihis last object is accomplished in accordance with `the present invention by opposing, in the tank circuit, any amplitude modulation inherently accomplished by phase modulation of the carrier, with equal and opposite modulation components furnished by the amplitude modulator described brieiiy in the second preceding paragraph.

Conversely, the

energy to the tank it is often desirable to same transmitter phase amplitude modulated that two signals may time, or to transmit vhum and extraneous phase Where multiplexing of signals by the useof v phase modulation and amplitude modulation is desired, the same carrier wave maybe modulated in phase by one signal to vbe transmitted, and in amplitude by another signal to be transmitted, or the same carrier may be modulated in phase and in amplitude by the same signal'. Moreover, the phase or amplitude modulators above need not be used simultaneously but may be used separately.

The novel features of the invention have been pointed `out with particularity in the claims appended hereto.

'I'he nature of the invention and the mode of operation thereof, and the advantages to be gained thereby will be understood by the following description thereof and therefrom when read in connection with the drawings attached hereto throughout which like reference numerals indicate like parts, and in which:

Fig, 1 shows by Way of illustration a phase and amplitude modulated transmitter which includes means by which the degree of phase modulation accomplished may be varied during operation of the transmitter and also means whereby the transmitter may be arranged to operate as a phase modulator or as an amplitude modulator, or as a phase modulator and amplitude mod-ulator for multiplex signalling; while Figures 2 and 3 showarrangements wherein means is provided for compensating the hum and extraneous phase modulation inherently produced in amplitude modulation of a carrier, and vice versa.

Referring tothe drawings, and in particular to Fig. V1 thereof, O indicates a high frequency oscillator of any known type such as a crystal oscillator which will produce oscillations of substantially constant frequency. The output of the oscillator O is connected, as shown, by way of blocking condensers I and 2 to the inductance 3 of a tank circuit 4 including a tuning condenser 6 connected in parallel with the inductance 3. The upper terminal of the tank circuit 4 is connected as shown by way of blocking condenser H and inductive reactance E to the grid 8 of a thermionic tube V1 and by way of a capacitive reactance F to the grid III of a thermionic tube V2. VThe lower terminal of the tank circuit 4 is connected by means of a blocking condenser I and variable resistance G to the controlV electrode I2 of an additional thermionic tube Vs. The resistance G is preferably non-inductive. The high frequency input circuit of the tubes V1, V2 and V3 is completed by way of a lead connecting a point on the inductance 4 to ground by way of capacity J1. l i

Audio' frequency signals from a source I4 Yare applied through transformer T1 and resistances R1 and R2 respectively in phase opposition to the control electrodes 8 and Ill'respectively of tubes V1, V2 respectively. The input circuit for the modulating signals'is completed by a lead I 6 connecting the center'point of the secondary windingof transformergT1 to a point o npotentiometerP, which is'in turn connected with the cath- 2,163,719 modulation which might be inherently produced ode circuit I8 of tubes V1, V2. Filament heating potential is supplied from the battery 20 as indicated. Biasing potential for the control electrodes 8 and I0 of tubes V1, V2 respectively is supplied by way of lead I6 from potentiometer P connected across a portion of the battery 2U.

The anode 22 of tube V1 and anode 23 of tube V2 are connected as shown to one terminal of a tank circuit 24 comprising an inductance 26 and Variable capacity 2'I. Charging potential for the anodes 22 and 23 is supplied by way of a lead 28 connected betweenY the battery 20 and a point on the inductance 26. Radio frequency oscillations appearing in this tank circuit are shunted around the source 2G by way of a bypass con- 1 denser 29. Y

The operation of this arrangement to accomplish phase modulation of the carrier supplied from O to the tubes V1, V2 in accordance with the signals supplied from I4 to said tubes has been described in considerable detail in the applications referred toV hereinbefore. The operation thereof, however, will be repeated briefly here.

The high frequency oscillations supplied from O appear relatively displaced or shifted in phase on the control electrodes 8 and I0 of the tubes V1, V2 respectively. The shift in phase depends of course upon the values-of the inductance L and capacity F inserted in the connections. Likewise,

due to the repeating effect of the tubes V1, V2 the phase of the energy in thecommon tank circuit 24 will be advanced or retarded from the average phase position of the normal radio Wave an angle, the sign of which depends upon which modulator tube is supplying the greater amount of power. The tube having lower instantaneous bias, `which is in part, the result of the signal oscillations differently applied to the control electrodes thereof byway of transformer T1, will, of course, have a greater controlling effect on the phase of the oscillations appearing in the common tank circuit 24. In effect, the phase of the oscillations appearing in the common tank circuit 24 will be advanced or retarded from the average phase position of the oscillations in a manner proportional to the controlling power of one modulator tube V1 over the other modulator tube V2. This controllingV power of one modulator tube V1 over the other modulator tube V2 of course depends upon the effective bias applied to the control electrodes of the respective tubes. Since the secondary winding of transformer T1 is energized by signal potentials the control electrode biasing excitation of the two phase modulator tubes will vary at signal frequency. The power supplied by each of tubes V1, V2 to the tank circuit will vary in accordance with the signal potential variations and therefore the repeated energy in the common tank circuit Will be varied in phase as the signal frequency applied to the transformer varies and will have an instantaneous phase which depends upon the effective bias applied to the control electrode, which bias is determined by the constant fixed potentials applied thereto by way of lead I 6 and the modulating potentials applied thereto from the secondary windings of transformer T1.

The phase modulated energy appearing in the tank circuit 24 may be supplied to= an amplifier 3!! by way of Vblocking condensersV 3l and 32. This amplifier may include frequency multiplying means and may be connected to an antenna system, as shown, from which the phase modulated energy may be radiated.

The means whereby amplitude modulations at signal frequency may be 'applied to the carrier from O will now be described.

The audio frequency source I4 has its output circuit connected by way of transformer T2, switch K, and resistance R3 to the control electrode l2 of tube V3 so that when switch K -is closed in the upper position modulating potentials are applied to the control electrode I2. This input circuit for the modulating potentials from I4 is completed by connecting the other terminal of the secondary winding' of transformer T2 to a tapped point on potentiometer P, which is in turn connected with the lament of tube V2 by way of filament circuit I8. This tapped connection between the transformer winding and the potentiometer permits the value of the biasing potential applied to the grid I2 of V3 to be regulated. The anode 33 of thermionic tube Vs is connected, as shown, to the lower terminal of tank circuit 24.

Assuming that the transmitter is operating as described above to accomplish phase' modulation of the carrier produced in O. If the switch K is closed (i. e., in the upper position) audio frequency oscillations are impressed from I4 upon the control electrode of tube V3 and amplitude modulated wave energy appears in the tank circuit 24 connected with the anode 33 of tube V3. These amplitude modulated oscillations are transferred along with the phase modulated signals to the amplifier 30 and radiated. The tank circuit 24 is normally tuned to the frequency of the tank circuit 4, both o-f these tank circuits being tuned to the frequency of the oscillations generated in O.

At the receiving station two receivers may be used or one receiver adapted to receive both audio frequency modulated waves and phase modulated waves and to separate the same, The carrier as transmitted by this arrangement has impressed thereon modulations in phase in accordance with the signals from source I4 and ymodulations in amplitude in accordance with signals from the source I4. Source I4 may be replaced by two sources as disclosed in my parent application Ser. No. 624,258 of July 23, 1932. 2

The amplitude modulator may be rendered inoperative by merely opening the switch K (down position) so that phase modulated signals only may be sent out.

In this arrangement any objectionable amplitude hum sometimes present in phase modulation transmission may be balanced out by applying from I4 potentials of a frequency equal to the frequency of the Yobjectionable hum but opposite in phase so thatthe supplied potentials will oppose the objectionable hum. In other words, any amplitude modulation accomplished in the phase modulator part of the transmitter may be opposed in the tank circuit by amplitude modulation accomplished in V3. By adjusting the direct current potential applied to the control electrode of tube V3 or by adjusting the amplitude of the potentials supplied from I4 to V3, the amplitude modulated signals applied from V3 to the tank circuit 24 may be madeequal in amplitude to the amplitude mo-dulation inherently accomplished in the phasey modulator part of the circuit and balance out the same so that pure phase modulated signals are fed from the tank circuit 24 to the amplifier 3B. Compensating potentials of the proper frequency phase and amplitude may be produced in I4 or supplied thereto from any source. Y

Conversely, where it is desired to transmit amplitude modulatedsignals entirely free of any hum and extraneous phase modulation which is inherently accomplished in the amplitude modulator, the amplitude'modulator tube V3 may be energized to accomplish full amplitude modulation of the carrier from O and feed the amplitude modulated carrier to the tank circuit. I4 may then supply current of a frequency equal to the frequency of the undesired phase modulation accomplished in the amplitude modulator V3. Thephase relation of this alternating current produced in I4 or supplied by way of I4 to V1 and V2 should be so adjusted that the phase modulation produced in the phase modulator will oppose and compensate the phase modulation produced in the amplitude modulator. The degree of phase modulation accomplished in the phase modulators V1 and V2 may be regulated by adjusting the value of the phase retarding and advancing means E and F respectively and, further, by regulating the direct current bias applied to the control electrodes` of the tubes V1 and V2 by way of resistances R1 and R2 respectively. Furthermore, the amount of current furnished by I4 to the phase modulator may be adjusted to a value such that the phase modulation components introduced into the tank circuit for correction purposes may be made equal and opposite to the undesired phase introduced into the tank tude modulator.

The manner in which the degree of phase modulation accomplished may be varied or adjusted will now be described.

If the key K is moved to the lower position so that the secondary winding of T2 is open at one end, no audio frequency signals from source I4 can be applied to the control electrode of tube V3. Now, the amplification of this tube depends upon the bias applied to the control electrode thereof. If the negative bias on the amplitudemodulator V3 is set high, tube V3 will supply only a small amount of unmodulated energy to the tank circuit 24 and the amount of phase modulated energy delivered to the tank circuit 24 from tubes V1, V2 will be relatively large. However, if the bias applied to the control electrode of the amplitude modulator V3 is set low, that is, less negacircuit from the amplitive, the amount of unmodulated energy delivered to the tank circuit 24 will be large and the amount of phase modulated energy delivered to the tank circuit 24 from tubes V1, V2 will be relatively small. The signal therefore in the first case will have a tion while in the second case the signal will have a small degree of phase modulation. In both cases, the transmitter output power will be the same or can be made the same ifa limiter is included in the ampliiier 30, but the degree of phase modulation can be made different. This arrangement therefore permits the degree of modulation in phase applied to the carrier from O to be Varied between certain limits. This variation of the degree of phase modulation may be accomplished during operation of the transmitter.

from one terminal and the control electrode of V3v modulation components large degree of phase modula- In order that the arrangement including thel only from the other terminal, some provision must be made to balance or equalize the load on this tank circuit. This resistor G permits this balance to be obtained.

In order to prevent the effects of coupling due to the inherent capacity of the electrodes in the tube from unbalancing the system, neutralizing condensers N1, N2 may be connected, as shown, between the terminals of the tank circuit 26 and the opposite terminals of the-tank circuit 4.

Although, for purposes of illustration, I have sho-Wn my novel modulating circuits as including thermionic tubes of the three-electrode type, it Will be understood that my invention is not limited to the use of anyspecifc type of tubes since screen grid or multi-grid tubes may be used in place of the tubes shown. Furthermore, the use of such tubes in place of the tubes now shown in the present invention is thought to be within the scope of the present invention. Where screen grid tubes are used in place of the triodes shown, neutralization of the tubes may not be necessary. Furthermore, Where screen grid tubes are used the modulating potentials maybe applied to the screening grid electrode rather than to the control electrode, as shown. Where multi-grid tubes are used, appropriate changes may be made Vin the circuit to derive from the use of said tubes the optimum results.

Although, as indicated above, the arrangement of Fig. 1 may be operated primarily as an amplitude modulator, the phase modulator therein serving to furnish phase modulated energy to balance out any phase modulation inherently accomplished in the amplitude modulator, in many cases itis necessary to have better control or regulation of the compensating energy than may be obtained with the arrangement of Fig. 1.

In Fig. 2 is shown aY preferred form of amplitude modulator means whereby the hum or extraneous phase modulation inherently accomplished in an amplitude modulator may be eliminated. In this arrangement audio frequency oscillations from the source I4 are supplied to an amplitude modulating potential amplifier 4U. The amplifier 40 is connected with an intermediate amplifier 4I to modulate in amplitude the carrier frequency appearing therein from the source O by Way of the phase modulator Q. The high frequency oscillations generated in O are impressed by way of a blocking condenser I, phase retarding means L, and amplitude regulating means R3 to the control electrode 8 of tube V1 and from' the blocking condenser I by way of phase advancing means M to the control electrode Ill of tube V2. These phase shifted oscillations produced as described in connection with Figure l are fed from the anodes 22 and 23'of tubes V1 and V2 respectively in parallel to the tank circuit 24 as in Fig. 1.

The tank circuit 24 is connected by way of blocking condensers 3l and 32 to the intermediate frequency amplifier 4I. The audio frequency source I4 is also connected by way of a potentiometer P1 and reversing switch S to the transformer T1, the secondary winding of which is connected as shown to the control electrodes 8.and I Il of tubes V1 and V2 by Way of resistances R1 and R2 respectively. In this manner audio frequency oscillations may be applied in phase opposition to the control electrodes of tubes V1 and V2. The amplitude of the oscillations applied to the phase modulators V1 and V2 may be adjusted by adjusting the potentiometer P1. 'The modulation potentials applied to the control electrodes of V1 and V2 may be reversed in phase by the reversing switch S. Energy for the filaments I8 of tubes V1 and V2 is supplied from a portion of direct current source 20, as in Fig. l, while biasing potential for the control electrodes of tubes V1 and V2 is supplied by way of a connection I6 between a point on the potentiometer P2, connected in parallel with a portion of the source 20, and the secondary winding 0f transformer T1. Charging potential for the anodes of tubes V1 and V2 is supplied by a lead 28 connecting the midpoint of the inductance 26 to the positive terminal of the source 2U. Charging potential forv the screen grid electrodes. of tubes V1 and V2 is supplied by way of a lead 33 connecting said electrodes to a point on potentiometer P3 connected in shunt with a portion of the source 20. Radio frequency oscillations appearing in the inputv vand output circuits and on the screen grid electrodes of tubes V1 and V2 are prevented from passing through the energizing source by bypassing condensers J2, J3, C connected Vas shown at appropriate points.

The operation of the transmitter as illustrated in Fig; 2 is as follows:

To begin with, assume that no amplitude modulation is present that is, that the modulation am- ,f

plier in 40 is inoperative. High frequency oscillations are generated in the oscillator O. This high frequency energy is repeated in phase modulator Q, Where it may be amplified or multiplied in frequency; when the oscillations are multiplied in frequency, 24 is tuned to a harmonic of the frequency of O, or both. Modulator stage Q may also be followed by a frequency multiplier or multipliers not shown. The high frequency oscillations are repeated in stages 4I and 43, where they receive further amplification. The oscillations amplied in stage 43 are delivered to the antenna F, where they are radiated.

Now, if audio signals are produced or amplified in I4, they will be transmitted to the modulation amplifier stage 40, which in turn modulates in any well known manner the amplitude of the radio frequency energy in 4I. The amplitude modulated signals or oscillations from intermediate amplifier 4I are repeated and amplified in 43 and transferred from 42 to the antenna F.

If in accomplishing the amplitude modulation in unit 4I no phase modulation has been caused, the movable point on potentiometer P1 should be moved to zero position so that no audio frequency is applied to the control electrodes of tubes V1, V2 of this phase modulation stage. The phase modulator Q will act as a straight amplifier or frequency multiplier of the oscillations impressed from O. However, if phase modulation is produced in the process of amplitude modulation in 4I, the procedure of eliminating it is as follows: P1 is adjusted so that audio frequency oscillations from I4 are applied to the control electrodes of tubes V1, V2, the carrier energy supplied from O` by Way of phase modulators V1, V2 to the intermediate amplifier 4I will be modulated in phase at the frequency of the oscillations from source I4. I'he reversing switch S should be put in the position which will cause the phase modulation produced in the tubes V1, V2 to oppose the phase modulation inherently produced in the amplitude modulator stages. After this has been done, potentiometer P1 may be adjusted until the phase modulation in the radiated signal is just balanced out.

Often the audio frequency potentials supplied from source I4 by Way of switch S are not of the proper phase to produce in the tank circuit 24 and following stages compensating potentials which oppose by 180 degrees the undesired phase vII) modulations inadvertently produced in amplitude modulating the carrier in 4I. For example, if when the switch S is closed in one position to furnish compensating phase modulations and a 45 degree phase relation exists between the phase modulation furnished from V1, V2 and the undesired phase modulations accomplished in the amplitude modulator, reversing the position of the switch S would merely shift the phase angle of the compensating energy to a different quadrant but still would not oppose the energies, that is, make them degrees apart.

In order to insure complete compensation of the undesired phase modulations produced in the amplitude modulator I provide a phase adjusting means 44 inserted as shown in the modulating potential leads between the source lll and the modulation amplifier 4D. This phase adjusting means permits the modulating potentials fed to the unit 40 to be adjusted to such a value that the phase modulation accomplished in 4I is exactly opposed by the compensating phase modulations produced in V1, V3.

The device of Fig. 3 is in the main similar' in arrangement and operation to the device of Figure 2. In the arrangement of Fig. 3, however, the audio frequency phase adjusting means 44 is interposed between the source I4 and the potentiometer P1. Here, asl in Fig. 2, the rough adjustment to oppose the undesired phase modulations produced in the amplitude modulator by phase modulations from the phase modulation stage is accomplished by the reversing switch S .while the more exact opposing adjustment is accomplished by the use of the phase adjusting means 4G.

If in the devices of Figs. 2 and 3 the undesired phase modulation in the signal is caused by oscillations of a frequency other than the frequency of the signal from I4, it is only necessary to introduce energy of this undesired frequency into the input circuit of phase modulators V1, V2 and by making the proper adjustments this undesired frequency will be eliminated from. the radiated signals.

While applicant has used three electrode tubes to illustrate the novel system of Fig. l, it will be understood that the invention is not to be limited thereby since applicant contemplates the use of any tubes known in the art in these systems. Likewise, the use of screen grid electrodes in Figs. 2 and 3 is not intended to limit applicant to the use of such tubes since, obviously, three electrode tubes may be used in place of the tubes shown. When three electrode tubes are used the system may be neutralized in a manner as illustrated in Fig. 1. Furthermore, the use of such tubes and the changes in the circuits necessary to derive the best results are within the scope of the present invention.

What is claimed is:

l. The combination, with an amplitude modulator comprising a modulator tube connected to a source of carrier frequency oscillations and a source of modulating potentials and an output circuit, of means for compensating any phase modulation inherently accomplished in said amplitude modulator including, a pair of thermionic tubes each having an anode, a cathode and a control grid, a connection between the anodes of said tubes and said modulator tube, circuits connecting the control grids of said tubes through phase shifting elements of different character to said source of carrier frequency oscillations, said pair of tubes being in said connection between said source of carrier frequency oscillations and said modulator tube, circuits connecting the grid to cathode impedances of said tubes in phase opposition to said source of modulating potentials connected with said amplitude modulator whereby phase modulation of said carrier frequency oscillations in accordance with said modulating potentials is accomplished in said tubes and circuits, and impressed o-n said amplitude modulator whereby the phase modulation inherently produced by said amplitude modulator is opposed and compensated by said other phase modulation.

2. Signalling means including, a work circuit, a source of high frequency oscillations, an amplitude modulator tube having an anode, a cathode and a control grid, a connection between said control grid and-said source of high frequency oscillations, a single source of signal potentials, means for applying potentials from said source of signal potentials to the control grid of said tube to produce amplitude modulation of said high frequency oscillations appearing in said work circuit, and means for balancing out any phase modulation accomplished in said amplitude modulator tube comprising, a pair of thermionic tubes each having anode, cathode and control electrodes, circuits connecting the control electrodes of said pair of tubes thro-ugh phase shifting devices ofV different character to said source of high frequency oscillations, circuits connecting the control electrodes of said tubes in phase opposition to said single source of signal potentials whereby compensating phase modulation of said high frequency oscillations in accordance with said signal potentials is accomplished in said pair of tubes and circuits, and means for impressing said last named phase modulation on said amplitude modulation to oppose and compensate said inherent phase modulation including circuits connecting the anode electrodes of said pair of tubes to said work circuit.

3. In a signalling system, a source of wave energy, a modulator, means for impressing' wave energy to be modulated thereon, a load circuit coupled to said modulator, a source of modulating potentials, and an amplifier coupling said source of modulating potentials to said modulator to amplitude modulate carrier wave energy therein, said wave impressing means including means for compensating hum or extraneous phase modulation inherently accomplished in said amplitude modulator comprising a pair of thermionic tubes each having electrodes including a cathode, a control grid and an output electrode, reactances of different character connecting the control grids of said tubes to said source of wave energy, a circuit coupling said source of modulating potentials in phase opposition to like electrodes in said tubes and means coupling the output electrodes of said tubes to said amplitude modulator, whereby compensatory phase modulation of said wave energy is produced in said tubes and said wave energy so modulated is impressed on said amplitude modulator wherein said phase modulations oppose and compensate said inherent hum or extraneous phase modulation.

4. A system as recited in claim 3 in which said last named circuit includes a reversing switch to reverse the polarity of said source of modulating potential relative to the said like electrodes of said tubes'.

5. A system as recited in. claim 3 in which said last named circuit includes potentiometer means for controlling the amplitude of the modulating potentials applied from said source to the said like electrodes of said tubes.

6. A system as` recited in claim 3 wherein said lo-ad Acircuit is tuned to a harmonic of the frequency of said source of wave energy.

7. A system as recited in claim 3 wherein said amplier is also a frequency multiplier.

8. A system as recited in claim 3 in which phase adjusting means is interposed in said last named circuit toy control the phase of the modulating potentials applied to said like electrodes.

9. A system as recited in claim 3 in which said like electrodes are the control grids of said tubes.

10. A system as recited in claim 3 in which phase adjusting means is interposed between said source of modulating potentials. and said amplier to control the phase of the modulating potentials impressed from said source on said amplier.

11. A system as recited in claim 3 wherein a Wave amplitude regulating resistance is connected in series with one of said reactances of diierent character connecting the control grids of said tubes to said source of wave energy.

12. In a signalling system, a source of oscillatory energy of carrier wave frequency, a source of modulating potentials, a work circuit, a relay stage having an output connected to said work circuit, said relay stage having an input, a pair of tubes having input electrodes and having output electrodes coupled to said relay stage, circuits connecting the input electrodes of said tubes to both of said'sources whereby the phase of said oscillatory energy is modulated in said tubes in accordance with said modulating potentials, means in said last named circuits for controlling the degree of said modulation, means for modulating in amplitude the input to said relay stage comprising an amplifier having controlling electrodes coupled to said source of modulating potentials and output electrodes coupled to said relay stage, and means connected with said source of modulating potentials to adjust the phase of the modulating potentials supplied to one of said modulators.

13. In a signalling system, a work circuit, an oscillatory energy amplier having an input and having an output coupled to said work circuit, a source of high frequency oscillatory energy, a source of modulating potentials, a pair of relay tubes each'having a control grid, a cathode, and an anode, phase shifting reactances of different character connecting the control grids of said tubes to said source of oscillatory energy, a tunable circuit connecting the anodes of said tubes together and to the input of said oscillatory energy amplifier, a modulating potential impedance connected between the control grids of said tubes, a

circuit including potential reversing means coupling said impedance to said source of modulating potentials, said arrangement being such as to produce phase modulation of said oscillatory energy in accordance with said modulating potentials, and means for modulating in amplitude the input to said amplier comprising an additional amplier having controlling electrodes coupled to said source of modulating potentials and output electrodes coupled to said oscillator energy amplier to modify therein said phase modulated oscillatory energy.

14. A signalling system as recited in claim 13 wherein phase and amplitude adjusting means is interposed in said coupling between said impedance and said source of modulating potentials,

15. A signalling system as recited in claim 13, wherein amplitude regulating means is interposed between said impedance and said source of modulating potentials and wherein phase adjusting means is interposed between said source of modulating potentials and the input of said amplitude modulator.

16. In signalling apparatus, a source of carrier wave energy, a single source of modulating potentials, an amplitude modulator connected to said source of carrier Wave energy, a circuit connecting said amplitude modulator to said source of modulating potentials, an output circuit connected with said amplitude modulator and means for balancing out any phase modulation inherently accomplished in said amplitude modulator comprising a phase modulator connected to said source of wave energy, a circuit connecting said phase modulator to said source of modulating potentials, and phase adjusting means in one of said circuits. f

17. In signalling apparatus, the combination of a source of carrier wave energy, a single source of modulating potentials, a phase modulator connected to said source of carrier wave energy and to an output circuit, a circuit connecting said phase modulator to said source of modulating potentials, and a device for balancing out any amplitude modulation inherently accomplished in said phase modulator comprising an amplitude modulator connected to said phase modulator, a circuit connecting said amplitude modulator to said source of modulating potentials, and phase adjusting means in one of said circuits.

18. A system as recited in claim 16 wherein one of said circuits also includes phase reversing means.

19. A system as recited in claim 17 wherein one of said circuits also includes phase reversing means.

GEORGE L. USSELMAN. 

